Mantle seismic tomography has historically relied on radially symmetric ID velocity
models to trace ray paths through the mantle. The resulting travel time residuals are used to
invert for seismic velocity perturbations around this 1D model. However, we know the Earth
deviates from such ID velocity models; for example there are global variations in crustal
thickness; in the age of oceanic lithosphere and presence of subducting oceanic lithosphere. In
light of this, an a priori model which incorporated the three types of surface observable
heterogeneity outlined above was constructed as part of this thesis. Tracing ray paths through
this more heterogeneous starting model resulted in new travel time residuals which were
subsequently employed in a simultaneous tomographic inversion solving for earthquake
relocation parameters and slowness perturbations. This inversion method allows us to
investigate whether tomography using a priori models results in improved images of mantle
velocity perturbations and systematic earthquake relocations.
A graphical earthquake browser was specifically written to establish, in a consistent
manner, the shape of subducting oceanic lithosphere for all the major subduction zones. The
resulting population of earthquakes, which best represent the shape of Wadati-Benioff zones,
were subsequently interpolated into profiles following the path of oceanic lithosphere as it
subducts. The temperature field in and around each profile was generated using a new
analytic solution of the heat equation for subducting lithosphere, adapted to incorporate slab
shape.
The upper mantle a priori model was constructed on an equal area tomographic grid
by combining the thermal models of the subducting lithosphere, plate cooling models of
oceanic lithosphere and variations in crustal thickness away from that prescribed in a ID
velocity model. Efficient 20 ray tracing through the a priori model was achieved via the
adaptation of a ID ray tracer by perturbing the reference ID model, iasp91, using the a priori
velocities in the cells connecting the event to the recording station for each ray. A new travel
time residual was calculated and subsequently used in the simultaneous solution for slowness
perturbation and earthquake relocation. So as not to bias the earthquake relocation procedure,
phases were selected so as to maximise the azimuth and epicentral distance coverage, while
minimising the number of duplicated ray paths which would be redundant in the inversion.
The data selection resulted in some 3,450 events emitting 785,000 teleseismic P phases
(bottoming in the lower mantle). The cell based SIRT inversion procedure, used to solve the
standard system of linear tomographic equations, was augmented by explicit damping and
smoothing matrices so as to control both poorly resolved cells and the relative importance
between earthquake relocation parameters and slowness perturbations. For comparison, the
ray population was also traced through the 3SMAC upper mantle model before undertaking a
similar inversion.
The 5° x 5° equal area, 100 km thick, cell inversions resulted in systematic
earthquake relocations with an average relocation distance of= 5 km. In the upper mantle, the
inversion procedure adjusts the a priori subducting slab velocity contrast, revealing images of
subducting oceanic lithosphere. In the lower mantle, there is little difference between
inversions produced in this thesis and those available digitally. Some of the main features are
the pronounced lineations interpreted as the Farallon slab (beneath North and South America)
and the Tethys (beneath Eurasia) clearly imaged between 1200 and 1500 km depth. All
inversions undertaken in this thesis image hotspots throughout the upper mantle, and in places
these pronounced slow features are observed passing through the upperllower mantle
transition. A section through the South Pacific superswell images slow material as a
continuous body, to at least 1300 km. Synthetic recovery tests indicate these hotspot features
are well resolved.